Determining the characteristics of Xanthomonas cucurbitae bacterium from the North Central Region and developing effective seed treatment for eradication of the bacterium in pumpkin seeds

Bacterial spot of cucurbits, caused by Xanthomonas cucurbitae, has become a serious threat to pumpkin production in Illinois and other states in the North Central Region (NCR) of the United States. This study was conducted to investigate the characteristics of X. cucurbitae isolates from the NCR and to develop an effective seed treatment to eradicate the pathogen in pumpkin seeds. Characteristics of the pathogen included the cell multiplication of X. cucurbitae at different temperatures and pH levels and pathogenic variation among isolates of the pathogen. Fourteen X. cucurbitae isolates from seven states (Illinois, Indiana, Iowa, Kansas, Michigan, Ohio, and Wisconsin) were tested. The American Type Culture Collection (ATCC) strain 23378 was included in this study as positive control. The result showed that the range of minimum, maximum, and optimum temperatures for cell multiplication of X. cucurbitae were 4-6, 34-36, and 24-30°C, respectively. Cell multiplication and colony development of X. cucurbitae was optimum at pH 6.5-8. For determining pathogenic variation among the isolates, X. cucurbitae was inoculated onto leaves of three-week-old ‘Howden’ and ‘Dickinson’ pumpkins in a greenhouse. Development of the bacterial lesions was recorded from 12 to 168 h post inoculation. The post-inoculation period for the first appearance of the visible lesions was significantly affected by the isolates. However, diameters of the lesions 168 h post inoculation were not significantly different among the isolates. Survival of X. cucurbitae in pumpkin seed was studied using naturally-infected and artificially-inoculated pumpkin seed stored at 4 and 22°C. X. cucurbitae survived longer than 18 months in the seeds at both temperatures and remained viable. The storage temperature significantly affected survival of X. cucurbitae in the seed. The effectiveness of hot-water, hydrochloric acid (HCl), and sodium hypochlorite (NaClO) for eradication of X. cucurbitae in pumpkin seed was tested. The results showed that a hot-water treatment at 55°C for either 10 or 15 min and HCl treatment at 0.5% concentration for 40 min eradicated X. cucurbitae in both naturally-infected and artificially-inoculated pumpkin seeds without any significant adverse effect on either seed germination or seedling vigor. None of the NaClO treatments eradicated X. cucurbitae in the seed